The interplay between various properties of materials in their non-superconducting state, such as xcfx81, the resistivity, and xcex3, and their superconducting properties such as Hc2, the upper critical field, has been examined intensively since 1959, beginning with the work of Gorkov, who first related the microscopic theory of Bardeen-Cooper-Schrieffer (BCS) to the phenomenological theory of Ginzburg-Landau (G-L), and extended significantly by Abrikosov to explain the properties of technologically important materials. This accumulation of knowledge is referred to as Ginzburg-Landau-Abrikosov-Gorkov (GLAG) theory and has been refined and extended by many investigators. The relationship between some important superconducting properties and normal state parameters was recently summarized by Orlando McNiff Foner and Beasley, (Physical Review, B19, p. 4545, 1979). Here it is shown that in Nb3Sn, a superconducting material which has been employed in the highest-field magnets currently in operation, the upper-critical field at, e.g., 4.2 K, can be significantly improved by increasing the resistivity of the material, albeit at the expense of a reduction in Tc. This reduction in Tc is likely responsible for the fact that the described increase in upper initial magnetic critical field has not received more serious attention. As far as can be determined, superconducting devices based on the A-15 compounds have not taken advantage of increased field values due to mixed cation occupancy.
A resurgence in interest in superconductivity worldwide is ascribable to the emergence of substituted copper oxide superconductors, the most significant of which show high critical temperaturesxe2x80x94many in a range amenable to liquid nitrogen cooling. Exemplary materials of this xe2x80x9cperovskitexe2x80x9d class are completely superconducting at temperatures in the range of 90-100 K (temperatures sufficiently removed from liquid nitrogen temperature to permit attainment of significant superconducting properties (Phys. Rev. Lett., Vol. 58, see, for instance, R. J. Cava et al, page 1676, and D. W. Murphy et al, ibid, page 1888).
This invention originated in the investigation of the properties of various examples of the phases described in a previous application. It was noticed that certain substitutions in the quaternary perovskite phases led to greatly enhanced high-field properties which are correlated to the increase in resistivity. While the GLAG formulation of the microscopic theory may not be obeyed in detail in these materials (e.g., if the superconductivity is not due to the usual electron-phonon interaction), the interplay between xcfx81 and Hc2 may nevertheless be qualitatively similar.
In contrast to work described in the xe2x80x9cBackgroundxe2x80x9d, it has been found that chemical substitutions made in perovskite copper oxide-based superconductors may give rise to increased values of the critical magnetic field for given current density or, conversely, increased current density for given values of magnetic field or combination of increased values of both. Unlike prior work reported under xe2x80x9cBackground of the Inventionxe2x80x9d, substitutions in accordance with the invention have no significant effect on the temperature of the onset of superconductivity (i.e., the value of Tc is not significantly changed).
The category of materials to which the invention applies is that of parent application, Ser. No. 07/024,026, (filed Mar. 10, 1987). The category of materials so encompassed and as described in the parent application is set forth under the Detailed Description herein.
The invention depends upon the finding that mixed occupancy, by M or Mxe2x80x2 elements within the general formulation M3xe2x88x92mMxe2x80x2mCu3Ox under the constraints set forth gives rise to an increase in critical magnetic field and/or current density under any given set of real operating conditions.
Simply stated, the inventive finding is to the effect that mixed occupancy in the xe2x80x9cAxe2x80x9d site, as described, gives rise to a critical field value increase for any given temperature. (The xe2x80x9cAxe2x80x9d site is occupied by the M and Mxe2x80x2 elements and refers to the conventional xe2x80x9cABO3xe2x80x9d designation for the primitive cell in the perovskite structure.) In terms of utilization this may take a variety of significant forms including:
1) Magnet structures capable of producing increased field,
2) Magnet structures of reduced size for a given required field,
3) Any other use in which superconducting properties are limited by magnetic field (circuitry might be subjected to a significant field due to proximity to a high field magnet or for whatever other reason. Stated differently, the inventive advance may be expressed in terms of higher permitted current density for any given magnetic environment.
For descriptive purposes, compositions of the invention are described in terms of prototypical compositions in which the A site in the xe2x80x9cpurexe2x80x9d compound consists of but a single divalent ion species, e.g., Ba, and but a single trivalent ion species, e.g., Y or Eu. Improvement in critical magnetic field of at least about,5% under given operating conditions corresponds with introduction of additional ions into the A site (into the M and/or Mxe2x80x2 location ) by an amount of at least about 1 at. % based on the total number of atoms in the A site. More preferred limitations correspond with critical field improvements of at least 10% corresponding with inclusion of about 2 at. % of third A site ions. Realization of critical field improvement at given operating conditions of 100% or more corresponds with mixed A site occupancy in which a third ion is included in amount of at least 10 at. percent based on the total number of (M and Mxe2x80x2) ions in the A site.
The invention consists of the optimization of the properties of quaternary cuprate superconductors by partial substitution. Nominal compositions may be represented by the nominal formula M2xe2x88x92yMxe2x80x21xe2x88x92zXy+zCu3Ox where M=Ba, Mxe2x80x2 is one of Y, Eu, or La, and X is at least one element different from M or Mxe2x80x2, and is one of elements 57-71 or Y, Sc, Ca or Sr. In general, significant increase in Hc2 corresponds with z+y values of from 0.3 to 1.0 with the provision that both M and Mxe2x80x2 be at least 50 at. % unsubstituted. The choice of substitution element X and amount z+y is dictated by the increase in resistivity which is sought. An important aspect of this invention is that, for many of the substitutions, the increase in resistivity does not come at the expense of a significant decrease in Tc as viewed in terms of a reference compound of unsubstituted M/Mxe2x80x2 composition in terms of majority M/Mxe2x80x2 atoms. Other variations in the unsubstituted compoundsxe2x80x94particularly variations from the nominal formulaxe2x80x94are set forth in the most recent parent application.
While the terminology used in the description is well known to the artisan, it is convenient to set it forth:
Hc1xe2x80x94Critical field value at which the Meisner effect is complete (magnetic flux is totally excluded).
Hc2xe2x80x94Critical field value above which all evidence of superconductivity is absent. (This term is meaningful in Type II superconductivity to which the invention is restricted in which real operation at values intermediate Hc1 and Hc2 correspond with superconducting conditions under which supercurrents are actually carried, even though the entire cross-section of the material may include local regions which are non-superconducting).
Tconsetxe2x80x94This is the temperature at which there is initial evidence of superconductivity, e.g., in terms of a significant change in slope of resistivity as a function of temperature.
TcR=0xe2x80x94The value of temperature at which there is a continuous path between applied electrodes such that the measured resistance between electrodes is zero.
Tcmidpointxe2x80x94The value of temperature which is equidistant between Tconset and TcR=0 in terms of resistance expressed.
Jcxe2x80x94critical current which, as in usual terms, defines the maximum current at which the sample is superconducting under given conditions (temperature and field).